Thermal, flow measuring devices have been used for a long time for measuring the mass flow of a liquid through a tube or pipeline. In such case, such devices rely essentially on two measuring principles.
In the case of the first measuring principle, a sensor in the liquid flowing through the pipeline is heated in such a manner that its temperature remains constant. Assuming that media properties, such as temperature, density and composition, remain the same, the mass flow of the liquid can be ascertained via the heating current needed to keep the temperature constant. In the case of variable media temperature, a second sensor is used for temperature measurement of the liquid.
In the case of the second measuring principle, the flowing liquid is heated with a constant heating power and the temperature of the liquid measured downstream. A higher mass flow leads to a lower measurement temperature and vice versa. Also in this case, a sensor for temperature monitoring of the liquid is helpful. For the heated sensors, reproducible manufacturing is important for achieving a high accuracy of measurement. If the solder layer thickness varies from sensor to sensor, then the required heating current also varies. In order that the flow measurement functions correctly, it is absolutely necessary that the temperature equilibration between sensor and liquid happens rapidly and that variations as regards this feature between individual sensors be as small as possible.
A typical sensor, in such case, includes a sensor thimble, in which a sensor element is arranged and thermally coupled with the sensor thimble by a solder layer. In such case, a uniform thickness of the solder layer between sensor thimble and sensor element is desired, in order to achieve a high accuracy of measurement. In the field of soldering, solutions are known, which lead to a small solder layer thickness variation, however, not to a satisfactory degree or in a manner disadvantageous for sensors of a thermal, flow measuring device. Thus, U.S. Pat. No. 4,626,478 discloses a subject matter with bulges, which assure a separation between the subject matter and a substrate. It is, however, very complicated to provide a sensor element with such bulges. The as yet unpublished patent application DE102016111701.3 avoids this problem by stamping a sensor thimble floor in such a manner that the resulting formations establish a defined separation of the sensor element from a face of the sensor thimble floor facing the sensor element. The stamping of the sensor thimble is, however, burdensome and is accompanied by wear of the stamping punch used therefor.